The present application relates to a battery pack, more particularly, to a battery pack applied to a non-aqueous electrolyte secondary battery, such as a lithium-ion secondary battery.
In recent years, various types of portable electronic devices, such as videotape recorders (VTRs) with camera, cell phones, and laptop computers, are widely used, and those having smaller size and weight are being developed. As the portable electronic devices are miniaturized, demand for battery as a power source of them is rapidly increasing. To reduce the size and weight of the device, the battery design in which the battery is lightweight and thin, and the housing space for the device can be efficiently used, is desired as well. It is known that, as a battery for meeting the demands, a lithium-ion secondary battery having a large energy density and a large power density is the most preferred.
In the development of the lithium-ion secondary battery, putting into practical use batteries having high selectivity of shape, sheet-type batteries having a reduced thickness and a large area, or card-type batteries having a reduced thickness and a small area is desired.
For meeting the demands, it has been recently reported that the thin battery as described above can be obtained using a casing member in a film form, such as an aluminum laminated film (see, for example,
TAKAMI Norio, “Ultrathin lithium-ion battery using aluminum laminated film case”, Toshiba Review, Toshiba Corporation, Vol. 56, No. 2, February, 2001, pp. 10-13 (hereinafter referred to as “non-patent document 1”)).
FIGS. 1A-1C show the external views of a battery 1 disclosed in the non-patent document 1. FIGS. 1A, 1B, and 1C are a plan view, a front view, and a section view of the thin lithium-ion battery for cellular phones, respectively.
The thin battery 1 is produced by covering a flat-type battery element, which is formed of stacking a positive electrode and a negative electrode through a separator and spirally winding them, with an aluminum laminated film and sealing the film around the battery element.
A positive electrode terminal 2a and a negative electrode terminal 2b respectively connected to the positive electrode and negative electrode are extended outside the battery, for example, from one side of the thin battery 1, and the aluminum laminated film around the battery element except for one side is sealed and then an electrolyte solution is injected from the unsealed opening, and finally the side of the film from which the positive electrode terminal 2a and negative electrode terminal 2b are extended is sealed, thus obtaining the thin battery.
This thin battery uses a casing of an aluminum laminated film having a thickness of about 100 μm, and hence has small strength, as compared to a battery using a metallic can, and it is difficult to use this battery as a battery pack as it is. Accordingly, a battery pack containing a battery element covered with a laminated film in a pack housing formed of a plastic and firmly fixed with an adhesive double-faced tape or the like is widely used. Further, it has been known that the battery pack can be improved in resistance to drop impact or shock by increasing the thickness of the housing.
However, in such a known thin battery, the housing is bonded with the surface of the battery at limited bonding portions, and hence, when the battery is dropped from a certain height or higher, there is a possibility that, although the battery element is not damaged, a load is concentrated on the flexible battery casing to break the casing or a pinhole is formed in the metal layer of the casing, and then moisture enters the inside of the battery by passing through the resin layer.
In this case, even in the battery pack which seems to suffer no deformation or the like from the external view of the pack and which can be fitted into an electric device, when the casing for battery is broken, moisture in air reacts with the electrode or electrolyte to continuously generate harmful gas, or when only pinhole is formed, the moisture vigorously reacts with the electrode or electrolyte to generate gas during the charging, so that the battery pack unusually expands, whereby causing a possibility of breaking the portable electric device.
Further, the pack housing inevitably has a thickness as large as about 300 μm or more due to the limitation of the technique for thin plastic molding or for securing the strength of housing, and therefore it is difficult to increase the battery volume to be contained in the housing.